Getting ActionController::UrlGenerationError in controller test after upgrade from 3.2.22 to 4.0.13

Hello.

While on 3.2 I had a controller spec which was calling an action:

get :show

The route was this:

guides_show GET /guides/:id(.:name.html)(.:format)

guides#show {:id=>/\d+/}

While I was missing the :id param, the test actually worked; it
successfully called the show action no matter the missing required
param.
Maybe this was not so correct behavior on behalf of Rails, but it was
like
that.

After upgrading to Rails 4.0.13, I now get the following expected
error:

Failure/Error: get :show
 ActionController::UrlGenerationError:
   No route matches {:action=>"show", :controller=>"guides"}
 #

/Users/as/.gem/ruby/2.1.5/gems/actionpack-4.0.13/lib/action_dispatch/journey/formatter.rb:40:in
generate' # /Users/as/.gem/ruby/2.1.5/gems/actionpack-4.0.13/lib/action_dispatch/routing/route_set.rb:597:ingenerate’
#
/Users/as/.gem/ruby/2.1.5/gems/actionpack-4.0.13/lib/action_dispatch/routing/route_set.rb:627:in
generate' # /Users/as/.gem/ruby/2.1.5/gems/actionpack-4.0.13/lib/action_dispatch/routing/route_set.rb:622:ingenerate_extras’
#
/Users/as/.gem/ruby/2.1.5/gems/actionpack-4.0.13/lib/action_dispatch/routing/route_set.rb:618:in
extra_keys' # /Users/as/.gem/ruby/2.1.5/gems/actionpack-4.0.13/lib/action_controller/test_case.rb:189:inassign_parameters’
#
/Users/as/.gem/ruby/2.1.5/gems/actionpack-4.0.13/lib/action_controller/test_case.rb:560:in
process' # /Users/as/.gem/ruby/2.1.5/gems/actionpack-4.0.13/lib/action_controller/test_case.rb:64:inprocess’
#
/Users/as/.gem/ruby/2.1.5/gems/actionpack-4.0.13/lib/action_controller/test_case.rb:472:in
`get’

If I add the id: 1 part in my test, it works fine (as it should).

While this change seems perfectly rational and more correct than the
behavior in Rails 3.2, I’m trying to find the actual commit/issue that
made
this change but can’t find anything (searched in CHANGELOG, rails
upgrade
notes, googled issues).
I’m not looking for a solution to my “issue” here, I’m just trying to
find
the actual change in Rails 4.0.x that changed the behavior.

Thanks!